1. FIELD OF THE INVENTION
This invention relates to an improved fire-retarding agent, to its use in connection with laminates composed of fibrous material, and to the finished articles themselves. More particularly, this invention is directed to improving the flame resistance of plastic laminates employed for printed circuits to improve their fire rating as established by industrial codes, to limit the afterburn time of ignited substrates by including in the substrates, together with an impregnating agent therefor, pentabromodiphenyl ether or a mixture thereof with other brominated diphenyl ethers.
2. DISCUSSION OF THE PRIOR ART
Plastic laminates may contain especially cellulose in the form of paper, continuous web or fiber layers as reinforcement. Such fiberboards are made by impregnating cellulose-paper webs with phenyl or cresol formaldehyde resols, with the addition of plasticizers as a rule, and hardening them in a hot press. The reinforcing, however, may also consist of slivers, laps, fabrics, mats or papers made of synthetic fiber forming materials such as polyesters or polyamides, or it may consist of glass continuous filament or glass wool. Laminates consist usually of a plurality of superimposed layers. They may be covered on one or both sides, in the same working procedure, with a metal foil, preferably copper foil, by the use of a hot glue, the purpose being to make electrical insulating material from this sandwich by known methods, to serve as supports for printed circuits, for example. Printed circuits are used in many electrical engineering and electronic applications, such as radios , television receivers, computers, etc.
For electrical applications it is particularly important that the fiberboard possess simultaneously good electrical insulating qualities, high mechanical strength and easy fabrication to printed circuits. Fabrication will be easy if the material can be cut and stamped at room temperature or with slight warming, and if the supporting material has good chemical resistance to solvents such as trichloroethylene, methyl ethyl ketone and lyes such as caustic soda solution, which are used in the manufacture of printed circuits.
Plastic laminates, however, may also be used as an interior decorating material, for example.
The following requirements in general must be met:
______________________________________ a. Insulation resistance after 4 days =&gt;10.sup.10 .OMEGA. 40.degree. C, 92% relative humidity b. Electrolytic corrosion effect on metals after 4 days, 40.degree. C, 92% relative humidity, DIN 53489 AN 1.4 c. Bending strength at 23.degree. C 1100 kp/cm.sup.2 d. Cuttability at 23.degree. C No cracking e. Stampability at 23.degree. C DIN 53488 Rating 2.5 f. Resistance to trichloroethylene vapor At least 5 minutes g. Resistance to 3 weight percent caustic soda lye at 40.degree. C At least 3 minutes ______________________________________
In addition, the material must be highly flameretardant for some applications.
Fiberboards are known whose flame resistance has been improved over that of ordinary phenolic resin fiberboards by the addition of flame-retardant substances.
The flame resistance of fiberboards has hitherto been judged on the basis of ASTM D 635 and ASTM D 229, Method 1, identical with NEMA LI-1, in which a test specimen measuring 12.7 .times. 102 mm is mounted with its long axis horizontal and its short axis inclined 45.degree. from the horizontal. A mark is placed at a distance of one inch from the free end opposite the mounting vise. With a bunsen burner whose blue flame has been adjusted to a length of 3/4 inch, the free end of the specimen is ignited twice for a period of 30 seconds. The afterburning time from the removal of the flame to the extinction of the fire in the specimen and the distance to which the specimen burned from the tip is measured and judged.
A phenolic resin fiberboard is considered to be flame-resistant according to this ASTM Test D 635 when, in the case of type NEMA Grade FR 2, the afterburning time is less than 15 seconds and the maximum length burned is less than one inch. There are a number of types of fiberboard, especially containing flame-retardant plasticizer additives, which satisfy these requirements.
Practice, however, has shown that materials of the said flame resistance are not sufficiently flame-retardant or are not self-extinguishing with sufficient rapidity to reliably limit an appliance fire, such as a fire in a television receiver, to the location of the circuit board in which electrical faults have occurred and thus the ignition has occurred.
If such materials catch on fire in printed circuit boards which are usually in a vertical arrangement, the chimney effect which occurs in the cabinet may cause them to burn up completely in spite of the above-described flame resistance, and the fire might spread to the back wall of the cabinet and often, too, to adjacent parts of the room.
The use of improved phenolic resin laminates which satisfy the above-described quality requirements have not yet made it possible for manufacturers to counteract the frequency of fires in recent times, especially in television receivers.
A method of testing flame resistance which will take into account the way in which these laminates burn in actual practice can be achieved by holding the specimen in the vertical position rather than the horizontal position over the bunsen burner during the test. This test method UL Subject 492, Paragraph 280 A-K, has been developed by Underwriters' Laboratories, U.S.A., and contains the following details.
A specimen measuring 1/2 inch by 4 inches long is mounted with its long axis vertical such that the bottom edge is 3/8 inch above the top of a bunser burner of 3/8 inch diameter. The burner is adjusted with a blue flame of 3/4 inch and held centrally beneath the bottom end of the specimen for a period of 10 seconds. The afterburning or afterglow is measured after removal of the flame. After complete extinction, the specimen is ignited a second time for a period of 10 seconds. The second afterburing or afterglow is also measured. Judgment of flame-resistant materials in accordance with this UL test with vertical burning is done according to two combustibility classes:
1. SE I (Self Extinguishing 1)
The average afterburning time must be equal to or less than 25 seconds. The maximum time may not exceed 30 seconds.
2. SE O (Self Extinguishing 0)
Here the average afterburning time must be equal to or less than 5 seconds and the maximum must not be greater than 10 seconds.
Material which complies with these more stringent tests and has a rating of SE I, or especially SE O, offers good passive fire protection for electrical appliances in which the insulation may be ignited in the case of trouble.
Ratings of SE I and SE O, however, can be achieved only by certain very special plastic laminates, and their composition and the additives they contain result in quality losses.
The SEO rating, for example, is achievable only on the basis of an epoxy resin, but cannot be achieved with phenol-cresol resins. Laminates bonded with epoxy resin have an incombustible glass fabric, unwoven glass fabric or cellulose paper for reinforcement. The epoxy resin is treated for flame resistance by the co-condensation of substances containing halogen, such as tetrabromobisphenol A. Partially for the purpose of enhancing flame resistance, antimony trioxide is also contained in the resin in proportions of 5 to 10% by volume.
The SE I rating can be achieved by fiberboards bonded with phenol-cresol resin, but only when either the resin or the plasticizer contains added flame-retardant substances, especially antimony trioxide. Then, however, the above-mentioned quality requirements are not achieved, and furthermore, other disadvantages result, which will be described hereinafter.
The use of mineral reinforcements or supporting materials involves difficulty in machining, because mineral substances cannot be punched easily and cause greater punch wear. Furthermore, the manufacture of such laminates is more expensive than laminates made with cellulose or paper reinforcements.
The use of antimony trioxide entails the disadvantage of making the laminate opaque, so that defects in the copper foil or conductors cannot be readily detected by inspecting them against a strong light. Furthermore, antimony trioxide, especially in large quantities, impairs the electrical and mechanical characteristics.
The addition of flame-retardant additives to phenolcresol resin solutions or to the plasticizers used therewith generally entails the following disadvantages:
1. The electrical characteristics are greatly impaired. Insulation resistance is diminished and in particular the electrolytic corrosion effect on metals is greatly intensified, so that the material cannot be used at all as an insulator.
2. The total quantity of platicizers and flame-retardants in the impregnating solution may not exceed approximately 35 to 40% by volume, because otherwise the resin hardens poorly and loses strength. If fireproofing agents are used, therefore, the quantity of plasticizers must simultaneously be reduced, but this gives the end product poorer stamping and cutting qualities. Embrittlement,too, is produced by the fact that numerous fireproofing agents decompose in the heating involved in the pressing procedure, thereby impairing the hardening of the resin.
3. The addition of flameproofing agents also results in the danger of rendering the impregnating solution turbid by the partial coagulation of plasticizers or resin components from the solution. This also impairs the hardening or setting of the resin and diminishes strength.
4. The flame retardants are incompatible or compatible only in small amounts with phenolic resin solutions and plasticizers, producing the effect described in Point 3.
Comparison Experiments 6 to 8 below show these disadvantages in detail when they are compared with Examples 1 to 5.
In view of the above it was heretofore generally accepted that it was impossible to provide a laminate with an SE O rating which laminate comprised a phenol-resol resin.
Specifically, those skilled in the art might have expected to achieve this aim with a mineral reinforcement and with copolymerizable flame retardant substances. However, it did not appear that it could be done with reinforcement on a cellulose base using known flameproofing agents. It was to be furthermore expected that there would be provided an impairment of the electrical, mechanical and fabricating qualities to the point where the material would be barely usable, if the difficulties described with regard to the manufacture of the laminates were overcome.
On account of the disadvantages listed as Points 1 to 4, supra, the art expected to obtain a usable flameresistant laminate with copolymerizable flame-retardant substances, but not with substances which remain in their molecular form.